This invention relates to parallel rules.
Parallel rules are used for navigation purposes to transfer a direction from one position to another on a chart. Thus a direction may be taken from a compass rose marked on the chart and drawn through a particular position on the chart so as to indicate a course or a transit, for example. When using navigational aids such as sextants or electronic global positioning systems (GPS) it is also necessary to mark a position where a line of latitude intersects a line of longitude, so as to show the craft""s position.
Against this background, in accordance with one aspect of the invention, there is provided a parallel rule, comprising two component rules each providing one of two opposed parallel straight edges, and a linkage allowing relative movement of the rules in a direction orthogonal to the straight edges but prohibiting relative movement in a direction parallel to the straight edges, wherein at least one component rule has at least one through hole suitable to receive the point of a pencil.
Such a rule may be used conventionally to mark a line of longitude through a particular easting or westing by placing one straight edge parallel to a line of longitude shown on the chart and expanding the rule until one or other straight edge crosses the particular easting or westing indicated at the edge of the chart. The rule may then be used to mark a northing by placing an edge adjacent the northing scale at the edge of the chart, so that the or a hole is positioned at the required northing, placing the point of a pencil in the hole and expanding the rule to draw a line of latitude on the chart at the required northing. In the alternative the line of longitude could be drawn analogously.
Most preferably, both component rules have through holes in corresponding positions. This allows one hole to be lined up with the appropriate northing and the pencil point to be inserted in the corresponding hole on the other component rule.
In one form the linkage may comprise an intermediate member connected to each component rule by a respective set of three links, two links in each set being pivotally connected to both the intermediate member and the component rule by pivots situated on corners of a variable parallelogram, the other link in each set being pivotally connected to the component rule and both pivotally and slidably connected to the intermediate member for sliding movement together, parallel to the straight edges.
In another form the linkage may comprise two links each pivotally connected to a respective component rule, each pivotally and slidably connected to the other component rule for sliding movement parallel to its straight edge and pivotally connected together between the component rules.
Another aspect of the invention extends to a parallel rule, comprising two component rules each providing one of two opposed parallel straight edges, and a linkage allowing relative movement of the rules in a direction orthogonal to the straight edges but prohibiting relative movement in a direction parallel to the straight edges, the parallel rule being contained by a package, the package being so formed that at least part of the parallel rule is visible therethrough, and so that the component rules may be opened and closed. This allows a prospective purchaser to operate the rule without removing it from its package.
In yet another aspect, the invention provides a parallel rule having two component rules so linked as to constrain them to remain parallel, wherein both component rules are provided on their undersides with a pair of spaced bosses, the bosses being of relatively low friction material, and between the bosses with at least one area of relatively high friction material, the bosses projecting further from the component rules than the high friction areas. As the component rules will be inherently a little flexible, pressing a component rule between the bosses, brings the relatively high friction material into contact with, say, a chart, firmly locating the rule thereon. Releasing the pressure raises the relatively high friction area from the chart so allowing the component rule to glide over the chart on the relatively low friction bosses. One component rule may thus be firmly located while the other is adjusted to its desired position.